In the conventional practice of applying a film of water paint having a specific thickness, the paint is first sprayed on the object to be painted and then the wet film thus formed is baked or dried.
When the water paint used has a low viscosity on account of low solid content and high solvent content the paint is easily atomized and yields a smooth film, but is likely to sag. Moreover, in time during baking or drying, the viscosity of the paint decreases when heated, which promotes sagging and at the same time increases any popping tendency which is likely to occur where the sagged paint collects.
On the contrary, when the water paint used is a high viscosity paint, on account of a high solid content and low solvent content, the smoothness of the film formed is poor but sagging is not liable to occur. Moreover during baking or drying, the decrease in paint viscosity due to heat is small; the smoothness of the film becomes poor and the sagging is less likely to happen.
Accordingly, a high-viscosity water paint, which gives poor smoothness of film, although it is free from sagging, spoils the appearance which is a commercially important factor of the product and is accordingly unfit for use. It is for this reason that adoption of a low-viscosity water paint is recommended to avoid the sagging defect of paint.